The invention relates to a device for the rationmetric measurement of sensor signals.
The demands on the measurement accuracy of sensors in motor vehicle electronics are becoming ever higher. Many measured values are transmitted from the sensor to the processing microcontroller as analog DC voltages Vmexcex2 (0 . . . 5V+0 . . . 100%). The microcontroller digitizes them using an analog/digital converter ADC (e.g. 10 bit ADC: 0 . . . 5V+0 . . . 1023 steps). In an analog/digital converter, the measured voltage Vmexcex2 to be converted is set in relation to a reference value VAref, which is supplied to the analog/digital converter ADC as an analog reference voltage. To convert the analog value to a digital value as accurately as possible, it is therefore desirable for the sensor S and the analog/digital converter ADC to use the same reference voltage.
In motor vehicle electronics, this is generally not possible because the reference voltage VAref used for the analog/digital converter ADC integrated in the microcontroller xcexcC is the supply voltage V5int for the microcontroller xcexcC (VAref=V5int). Since, however, in the event of a short circuit between the sensor reference voltage V5ext and ground or the battery, the microcontroller and hence the engine/transmission control unit need to remain operational, the supply voltage V5int must not be routed to the vehicle wiring harness.
In this context, the notation V5int signifies the internal 5 volt supply voltage for the microcontroller xcexcC, while V5ext signifies the 5 volt reference voltage for the sensor or sensors S arranged outside of the motor control unit ST. The supply voltage for microcontroller and sensors is usually 5 V.
A voltage controller SR having a plurality of mutually independent outputsxe2x80x94V5int, V5extxe2x80x94and a common reference Ref is generally used, see FIG. 1. The mutual discrepancy (tracking error of the controller) which is possible in this context is typically V5intxe2x88x92V5ext=xc2x150 . . . 100 mV. In sensors such as air mass flowmeters, this error (1 . . . 2%) is already an essential proportion of the total error budget.
It is therefore the object of the invention to provide a device for measuring sensor signals which can be used to eliminate or minimize this tracking error.
In the device according to the invention, the reference voltage VAref for the analog/digital converter ADC is isolated from the supply voltage V5int for the microcontroller xcexcC. It followsxe2x80x94in a certain range around the supply voltage V5intxe2x80x94the sensor reference voltage V5ext, so that the tracking error is largely eliminated in this range. This range is defined by the maximum permissible difference between the reference voltage VAref for the analog/digital converter ADC and the supply voltage V5int for the microcontroller xcexcC; in a typical microcontroller used in automobile electronics, it is xc2x1100 V, for example.
An illustrative embodiment in accordance with the invention is explained in more detail below with the aid of a schematic drawing, in which: